A free-piston Stirling refrigerator for generating cold heat is also called reverse Stirling refrigerator in terms of heat cycle. This Stirling refrigerator has a structure as described below with reference to FIG. 12.
A conventional Stirling refrigerator 100E has a cylinder 3 including a linearly reciprocating piston 1 and a displacer 2. Piston 1 and displacer 2 are coaxially structured and a rod 2a formed on displacer 2 passes through a slide hole 1a provided in a central part in the axial direction of piston 1. Piston 1 and displacer 2 are provided to be smoothly sidable along an inner-periphery slide surface 3a of cylinder 3.
At an upper part (on the right side in FIG. 12) of rod 2a formed on displacer 2, respective central parts of a piston support spring 5 and a displacer support spring 6 are fixed. Piston support spring 5 and displacer support spring 6 are each in the shape of a spiral disk-like panel.
Piston 1 is elastically fixed with respect to casing 15 by piston support spring 5 supported by a support member 31 fixed to casing 15. Displacer 2 is also elastically fixed with respect to casing 15 by displacer support spring 6 supported by support member 31.
The internal space formed by cylinder 3 is divided into two spaces by piston 1. A first space is a working space 7 formed at the side of displacer 2 with respect to piston 1. A second space is a back space 8 formed at the opposite side of displacer 2 with respect to piston 1. These two spaces are filled with such a working medium as helium gas at high pressure.
A linear motor 16 includes an inner yoke 13 fixed to cylinder 3, an outer yoke body 9 formed of an outer yoke 9b placed with a predetermined gap between itself and inner yoke 13 to enclose a bobbin/coil 9a, and a permanent magnet 12 attached to piston 1 and placed in the gap between inner yoke 13 and outer yoke 9b. Outer yoke 9b is fixed to casing 15 by a positioning block 30 supported by support member 31.
Piston 1 is axially reciprocated at predetermined cycles by the action of linear motor 16. The reciprocating motion of piston 1 causes the working medium to be repeatedly compressed and expanded in working space 7. Displacer 2 is linearly reciprocated by a change in pressure of the working medium which is compressed and expanded in working space 7. Piston 1 and displacer 2 are configured to reciprocate at the same cycles with a phase difference therebetween of approximately 90°.
Working space 7 is further divided into two spaces by displacer 2. A first working space is a compression space 7a located between piston 1 and displacer 2. A second working space is an expansion space 7b at the top of cylinder 3. Compression space 7a and expansion space 7b are coupled via a regenerator 4. Regenerator 4 is formed of a mesh-shaped copper member for example.
The working medium in expansion space 7b generates cold heat at a cold head 3c at the top of cylinder 3. Reverse Stirling heat cycle such as this principle of generation of cold heat is a well-known art and thus description thereof is not provided here.
Stirling refrigerator 100E of the above-discussed structure, however, has following problems.
First, components of coil/bobbin 9a and outer yoke 9b have low strength and thus these components must be handled carefully in assembly of mass production. Second, in the structure as shown in FIG. 12 with piston support spring 5 and displacer support spring 6 fixed to casing 15, support member 31 fixed to casing 15 has to be extended to the positions of piston support spring 5 and displacer support spring 6, resulting in increase in size of the outer shape of casing 15 to make it necessary to increase the thickness of a material for casing 15 in terms of strength.